1. Field of the Invention
This invention relates to Global Positioning System or GPS receivers.
2. Description of the Prior Art
GPS systems were originally developed to provide position information and navigation data for the military. Military uses require that the full accuracy of the provided data not be easily available to opposing forces. Many techniques have been incorporated into the GPS system for limiting access to the information including the use of spread spectrum transmissions, multiple levels of pseudorandom modulation encoding and Selective Availability (SA) techniques.
Non-military GPS systems have been widely accepted and used. Non-military, or more generally, unauthorized users do not have access to the full capabilities of the GPS system. Access is granted to authorized users by distribution of a GPS `key` or cryptovariable. Only access to this key allows the authorized user to have access to the classified satellite position and range information. The GPS position, when formed as a point solution from the classified satellite position and range information, is unclassified, and can be provided as the unclassified receiver output. This can allow a military GPS receiver to remain unclassified when keyed. Of concern to the military, however, is how to limit access to the classified data when a GPS receiver which has been loaded with a GPS `key` falls into the hands of an unauthorized user.
One proposed technique for protecting the secure information is the proposed Selective Availability Anti-Spoofing Module, or SAASM-based receiver. The specification for SAASM is SS-GPS-001, Global Positioning System (GPS) Selective Availability Anti-Spoofing Module (SAASM) System Specification, dated Dec. 5, 1994. The unclassified SAASM receiver maintains all secure information internally and provides only unclassified outputs, including the permitted point solution information. One expected difficulty with such security systems is that other user equipment, such as inertial navigation systems and the like, are designed to utilize the satellite position and range information.
The purpose of the SAASM configuration is to limit the availability of GPS-classified data outside of the tamper-resistant security module. Certain user equipment, such as tightly coupled GPS-Inertial integration requires the corrected pseudorange and delta-range observations, which are classified. Various alternatives have been proposed to accommodate these conflicting requirements, including incorporation of the integration filter inside the SAASM (which greatly increases the complexity of the SAASM), use of a Position/Velocity/Time solution in the integration filter (which eliminates the widely recognized advantages of tight coupling) and development of a classified SAASM which outputs classified pseudorange and delta-range (which is antithetical to the concept of SAASM).
What is needed is a technique for using secure receiver systems such as unclassified SAASM receivers with user equipment requiring satellite position and range information without defeating the security provisions of the SAASM receivers.
It should be noted that the identification of classified vs. unclassified data made within this application represents the best understanding of the author at the time its writing. The sole authority for classification determination for GPS data is the GPS Joint Program Office, and such determinations are subject to interpretation and change.